This invention relates to a method for continuously measuring the sulfite to sulfate concentration ratio in a process solution. More particularly, the method involves determining the difference between oxidation potentials from a measurement of the potential difference between electrodes disposed in an absorbent and a reference solution. Even more particularly, the present invention is useful in monitoring and controlling the oxidation operation of the flue gas desulfurization processes.
In a wet flue gas desulfurization (WFGD) processes, SO.sub.2 is absorbed and converted to a sulfite ion, which is further oxidized to a sulfate ion before being precipitated as CaSO.sub.4. That is to say, in a wet FGD process, solutions containing sulfite ions are produced. These sulfite containing solutions are then oxidized so as to convert a major portion of the sulfite ions in the solution to the sulfate ion form. This sulfate form is then mixed with a slurry containing Ca(OH).sub.2, (that is, a lime slurry). This results in the formation of CaSO.sub.4 which is formed as a crystal. This crystal precipitate is conventionally known as gypsum. It is disposed of or sold as construction material.
The oxidation of the sulfite containing solution to a sulfate solution is carried out by aerating the solution with an oxygen containing fluid, such as air. However, excessive aeration should be avoided because aeration consumes energy and excessive aeration destroys additives which may be present in the absorbent solution. The extent of this oxidation, particularly as measured by the ratio of the concentration of sulfite ions, [SO.sub.3.sup.-- ], to the concentration of sulfate ions, [SO.sub.4.sup.-- ], should be the one which gives the highest yield of gypsum but minimizes the energy consumption and organics destruction. These two concentrations are critical parameters which determine the performance and economics of the sulfite oxidation operation of the flue gas desulfurization process. Accordingly, it is seen that it is highly desirable to be able to determine the [SO.sub.3.sup.-- ]/[SO.sub.4.sup.-- ] ratio so as to monitor and control this aspect of the desulfurization process.
For proper control of the oxidation process, it is desirable to be able to measure this ratio on a continuous, rather than a batch basis. Currently, this ratio can be determined by chemical titration of the solution or by analyzing each ion species with ion chromatography methods. However such methods are not continuous and require separate analysis of each sample which is taken. In addition, ion chromatography requires expensive equipment. Such methods are not conveniently applicable for on-line control of the flue gas desulfurization process. This gap in measurement and instrumentation technology has however, been filled by the present invention.